This invention relates to a thermally integrated laser rangefinder and more particularly to an apparatus which combines a forward looking infrared (FLIR) system for locating and identifying targets and a laser system for determining the range to the targets.
A typical rangefinding system includes a coherent radiation source, such as, for example, a Nd:YAG Laser, which produces a pulse of energy which is transmitted to the target, reflected, and then received at the point of transmission. The total travel time is the measure of range to the target. The rangefinder is typically pointed at the target using information from a thermal imaging sensor, such as a FLIR.
The problems with the prior systems are many. In particular, the energy transmitted by a Nd:YAG Rangefinder under adverse weather conditions is attenuated much more than the longer wavelength energy used by the FLIR and conditions can be such that a target can be seen but not ranged upon. This compatability problem relates primarily to the spectral match between the FLIR and laser rangefinder.
The use of a FLIR system to locate a target and a separate laser rangefinder system to range the target includes the duplication of many parts because of the spectral match. The spectral match can be improved by using a longer wavelength laser transmitter. Nevertheless, the detection of the return energy from a long wavelength laser such as, for example, a CO.sub.2 laser requires a receiver system having larger optics of more expensive materials to transmit and receive diffraction limited beams, and also requires a detector which needs cryogenic cooling to achieve adequate detector sensitivity. Such a system is very costly.
A FLIR system includes the types of components which are required by the receiver of the long-wavelength rangefinder. Under conditions where ranging is to be performed, the FLIR uses a large aperture collecting lens system which focuses energy from the target onto a sensitive, cooled detector. This portion of the FLIR therefore duplicates the function which must be performed by the laser rangefinder receiver. Thus, from a cost effectiveness viewpoint the basic FLIR components should be utilized for the detection of laser pulses. But, this has not been achievable directly because of the difference in design requirements between a FLIR system and a laser rangefinder.
The FLIR is an imaging sensor, using an array of detectors to scan a large area; the target is only a small portion of the scanned area. Also sensitivity is achieved in the FLIR in part by operating each detector at the lowest possible information rate to decrease the noise bandwidth of the system. The rangefinder uses short duration pulses the require wide receiver bandwiths to achieve adequate range resolution.
In addition, a rangefinder generally has a fixed axis while a FLIR requires a rotating or reciprocating axis for scanning the FLIR detector. The FLIR detector is a low frequency detector and processor, while the laser rangefinder detector is a high frequency detector and processor.
The above enumerated differences thus have led to the development of FLIR system designs which are independent of range finding requirements.
In addition, the combining of the laser and FLIR systems to form a cost efficient integrated rangefinder presents many other problems. For example, in a sighting device, having a visible energy optical channel and an infrared (IR) optical channel for a gunner and a commander, expensive sight reticles are required for each telescope in the visible energy optical channel and each IR display in the IR energy optical channel. Another problem attending the use of the visible energy optical channel for projecting the laser beam is the alignment of the laser with the visible energy optical channel. Still another problem is the need for a laser transmitter compatible with the range of the FLIR and/or weapon. Yet another problem is the monitoring of the laser firing and the synchronization of laser firing with the position of the FLIR's scanner. Yet still another problem is to utilize the FLIR to detect laser returns and provide spike type signals for the timing function.